The present invention relates to an apparatus and method for interrogating a passive sensor and, particularly, but not exclusively, to an apparatus and method for interrogating a passive sensor incorporating a Surface Acoustic Wave (SAW) device.
It is well known for Surface Acoustic Wave devices to be used for monitoring parameters such as temperature, pressure and strain. Such devices are comprised of a resonator made up of a micro-structure deposited on a piezoelectric substrate. The deposit is arranged to form at least one pair of interleaved comb like electrodes. Typically, the electrodes are made from aluminum (although other good conductors may be used) and have a thickness in the order of 100 Angstroms. The piezoelectric substrate is typically manufactured from ceramic or quartz material.
In use, the application of an electric pulse signal to one electrode of an electrode pair causes the associated SAW device to act as a transducer. The electric input signal is converted to an acoustic wave which is transmitted via the substrate to the other electrode of the electrode pair. On arrival of the acoustic wave at said other electrode, the transducing process is reversed and an electric output signal is generated. This output signal has a characteristic resonant frequency which is dependent upon a number of factors including the geometry of the micro-structure deposit (e. g. the spacing between the interleaved electrodes).
Since the spacing of the interleaved electrodes within an electrode pair varies as the substrate is deformed (e. g. by virtue of temperature variations or the application of a mechanical force), a change in the condition of a SAW device can be determined by monitoring the device resonant frequency. In this way, parameters such as temperature, pressure and strain which affect the condition of a SAW device may be measured. For example, if the temperature of a SAW device is reduced, then the piezoelectric substrate reduces in size and the interleaved electrodes move closer together. As a result, the resonant frequency of the SAW device increases. This increase can be measured and calibrated to provide an indication of the temperature change.
In prior art systems for interrogating a SAW device, an input signal in the form of an excitation pulse (typically having a period of 10 xcexcs) is applied to one of the device electrodes by interrogation apparatus so as to initiate resonance.
Following termination of the excitation pulse, the output signal generated by the SAW device progresses through a transitional phase before stabilising for a short period of time at the resonance frequency. In the time taken for the output signal to stabilise, the interrogation apparatus switches from a signal transmitting mode to a signal receiving mode. The output signal from the SAW device may be then received by the interrogation apparatus and appropriately analysed so as to determine the output signal frequency. The frequency of the output signal is equal to the resonant frequency of the SAW device.
However, the need to allow the output signal from the SAW device to stabilise and the need to switch the interrogating apparatus between transmitting and receiving modes can unduly prolong an interrogation procedure, especially in circumstances where a large number of passive sensors are to be interrogated by the same interrogating apparatus. This is a perceived problem with the prior art systems.
It is an object of the present invention to provide an apparatus and method for determining the resonant frequency of a passive sensor in a convenient and rapid manner.
It is also an object of the present invention to provide an apparatus and method for interrogating two or more passive sensors functioning under different operating principles.
A first aspect of the present invention provides apparatus for determining the resonant frequency of a passive sensor, said apparatus comprising: means for transmitting signals of a plurality of frequencies to said passive sensor; means for receiving signals from said passive sensor simultaneously with said transmitting of signals; means for comparing the received signals with the transmitted signals to identify a characteristic change in the relationship between the received signal and the transmitted signal which occurs when the sensor is being driven at its resonant frequency; and means for determining the frequency of the transmitted signal at the moment when the comparison indicates that the sensor is being driven at its resonant frequency.
Thus, apparatus according to the present invention transmits a driving signal to a passive sensor such as a SAW device and at the same time receives the signal generated by the sensor in response to the driving signal. The return signal received by the interrogating apparatus is then compared with the transmitted signal. The transmitted signal and received signal will have a phase relationship determined at least in part by the mutual spacing of the transmitter, the received and the sensor. For any particular spacing, the phase relationship will depend on the phase relationship between the signal driving the sensor and the signal generated by the sensor in response to the driving signal. This phase relationship between the driving signal and the signal produced by the sensor in response to the driving signal will change at the resonant frequency of the sensor.
By comparing the received signal with the transmitted signal as the frequency of the transmitted signal varies but the distance between the transmitter, receiver and sensor remains substantially constant the phase change in the signal generated by the sensor in response to the driving signal can be detected. The frequency at which the transmitter is driving the sensor at the moment when resonance is detected will be the resonant frequency of the sensor and accordingly when resonance is detected as described above the resonant frequency can be obtained directly by identifying the frequency at which the transmitter is driving the sensor.
Preferably the driving signal is a radio frequency signal which is frequency modulated with a sweeping range of frequencies which span the expected resonant frequency of the sensor. Preferably the signals are transmitted across a gap between the transmitter and the sensor as radio signals by means of an antenna associated with the driving/comparing system and an antenna associated with the sensor. Preferably, a single antenna is used in the driving/comparing system for both transmitting the driving signal to the sensor and receiving the resultant signal from the sensor. Preferably the comparison of the transmitted and received signals is effective using a homodyne receiver.
A second aspect of the present invention provides a method for determining the resonant frequency of a passive sensor, said method comprising: transmitting signals of a plurality of frequencies to said passive sensor; receiving signals from said passive sensor simultaneously with said transmitting of signals; comparing the received signals with the transmitted signals to identify a characteristic change in the relationship between the received signal and the transmitted signal which occurs when the sensor is being driven about its resonant frequency; and determining the frequency of the transmitted signal at the moment when the comparison indicates that the sensor is being driven at its resonant frequency.